In this technical field, various research and development efforts have been made at a rapid rate on the themes of a next-generation mobile communication system including a radio access system, re-transmission control, and handover.
FIG. 1 is a sequence diagram showing one example of a re-transmission procedure. In FIG. 1, the right-hand side shows the operations on a transmitting side of the entities of the Medium Access Control (MAC) sublayer (TxMAC) and the Radio Link Control (RLC) sublayer (TxRLC); and, the left-hand side shows the operations on a receiving side of the entities of the MAC sublayer (RxMAC) and the RLC sublayer (RxRLC). In downlink transmission, a base station serves as a transmitter and a user equipment (UE) serves as a receiver. On the other hand, in uplink transmission, a base station serves as a receiver and a user equipment (“UE”) serves as a transmitter.
In steps S21 and S22 in FIG. 1, a packet to be transmitted is provided. Namely, the MAC sublayer sends a request (new data request) to the RLC sublayer to provide the packet to be transmitted. To respond to this request, the packet to be transmitted is provided in the MAC sublayer. In the example of FIG. 1, a sequence number “0” (SN=0) is added to the packet, namely a packet data unit (PDU).
Next, in step 11, the packet provided in the transmitting side is transmitted to the receiving side. In this transmission, the packet data unit including user data defined by the sequence number (SN) is transmitted through a data channel; the user equipment identification information (UE-ID), the process number (Proc), the New Data Indicator (NDI), and other control information items are transmitted through a control channel; and, as the system frame number (SFN) which indicate the absolute transmission timing in a cell, data broadcasted through the broadcast channel are used.
The receiver that received the control channel, the data channel, and the broadcast channel performs error detection by using a Cyclic Redundancy Check (CRC) method or the like with respect to the received packet. The error detection shows a negative (NACK) or affirmative (ACK) result. The former (NACK) denotes that an error beyond an allowable range is detected; and the latter (ACK) denotes that an error beyond the allowable range is not detected. In the case of FIG. 1, an error is detected (CRC:NG).
Next, in step S12, the error detection result is transmitted to the transmitting side. In response to the “negative” error detection result, the transmitting side identifies the packet that is related to the negative result, and re-transmits the identified packet. To make it possible to identify the packet, each of the packets radio-transmitted from the transmitting side is stored in a buffer (re-transmission buffer) after the transmission and discarded when a received response indicates that the error detection result is affirmative (ACK). By using this feature, upon receiving a negative error detection result, the packet transmitted before is identified in accordance with the result and re-transmitted.
As shown in step S12 of FIG. 1, a negative result (NACK) is transmitted to the transmitting side. Therefore, the transmitting side should appropriately re-transmit the packet upon recognizing the negative result. However and unfortunately, depending on the radio link conditions, the transmitting side (TxMAC) may incorrectly recognize the detection result, namely, the transmitting side may recognize as if an affirmative detection result were reported. Similarly, in the example of FIG. 1, even though a negative result is responded from the receiving side, the transmitting side continues its processes as if an affirmative result were reported from the receiving side.
As a result of this incorrect recognition, the subsequent different packet is provided in steps S23 and 24 and transmitted from the transmitting side (transmitter) to the receiving side (receiver) in step S13. In this transmission, the packet having SN=4 along with the control data of Proc=0 and NDI=1 is radio-transmitted at the timing SFN=8.
The receiver checks the received process number (Proc) and the new data indicator (NDI) and recognizes that it is not a re-transmission packet but a new packet that is transmitted even though a negative result has been previously reported to the transmitter. As a result, the receiver may recognize that the processes are being continued based on the wrong recognition in that the negative result is incorrectly recognized as an affirmative result.
Next, in step S14, in response to the error detection result, an indicator including the information indicating the system frame number (SFN=3) of the packet to be re-transmitted is provided and transmitted to the transmitting side. An object to be re-transmitted is the packet related to the negative error detection result and determined by the result of the error detection performed after step 11 and the relevant SFN. The indicator may be referred to as a False Ack Indicator (FAI).
The transmitter extracts the system frame number (SFN=3) from the reported indicator. The sequence number of the packet transmitted at the system frame number (SFN=3) is “0”. This result is obtained based on the data stored in the transmitter side. By doing this, the sequence number of the packet to be re-transmitted is identified based on the reported system frame number.
Next, in step S25, the identified sequence number (SN=0) is reported to the TxRLC sublayer that manages the transmitted packets. Then, the packet having the sequence number (SN=0) is re-transmitted to the receiving side. To simplify the figure, any further steps following step S26 are not depicted in FIG. 1.
As described above, even when incorrect recognition of the error detection result (ACK/NACK) occurs in the transmitting side, the receiving side may identify the packet to be re-transmitted, thereby enabling performing appropriate re-transmission control. A method like this is described in, for example, R2-060907, “MAC functions: ARQ”, Samsung.